A significant amount of bitumen in Alberta, Canada and other parts of the world is located either in thin, bottom-water reservoirs or water-sensitive sands that are not amenable to exploitation by steam-based processes. Potential alternatives to extract heavy oil from these reservoirs are solvent-dominated processes, sometimes referred to as Diluent-Based Recovery (DBR) processes. The advantages of the solvent-dominated processes include significant reduction in greenhouse gas emissions, little heat loss, and limited water handling. The disadvantages of the solvent-dominated recovery processes include high solvent cost and inherently low production rates limited by mass transfer of the solvent into the heavy oil.
In general, many processes and methods utilizing a variety of solvents/diluents under a variety of temperature and pressure conditions have been developed to improve solubilization and production of hydrocarbons from reservoirs.
Lim et al. in Canadian SPE/CIM/Canmet International Conference on Recent Advances in Horizontal Well Application, Mar. 20-24, 1994, discloses the use of light hydrocarbon solvents to produce bitumen for Cold Lake (Alberta) oil sands in three-dimensional scaled physical modeling experiments. Lim et al. discloses that the production rate of bitumen was significantly higher than what could be expected from molecular diffusion of the solvent into the bitumen. Lim et al. surmised that other mechanisms, probably solvent dispersion or fingering, are important in mass transfer of solvent into bitumen.
Lim et al. (1995) in Society of Petroleum Engineers paper no. SPE 302981 p. 521-528 discloses cyclic stimulation of Cold Lake oil sands with supercritical ethane through a single horizontal injector/producer well in a model system. Supercritical ethane enhanced the cyclic solvent gas process by improving the early production rate. SPE 302981 directs the reader towards using supercritical ethane.
Canadian Patent No. 2,349,234 discloses a Cyclic Solvent Process (CSP) for heavy oil production involving injecting a viscosity reducing solvent into a reservoir at a pressure above a liquid/vapor change pressure of the solvent, allowing the solvent to mix with the heavy oil under pore dilation conditions, and then reducing the pressure to below the liquid/vapor change pressure, thereby causing solvent gas drive of the solvent from the reservoir.
In addition to relying on the choice of solvent and pressure, heat has also been introduced into the reservoir to reduce the viscosity of the heavy oil, thereby enhancing the flow and recovery of heavy oil. The introduction of heat also results in the suppression of the formation of a second liquid phase that is often formed when solvent at low temperature is mixed with heavy oil whereby the heaviest of the heavy oil constituents (asphaltenes) resides in a heavier layer and a solution of the lighter components in the solvent forms a separate upper layer. The heavier layer creates a gummy residue that may potentially clog up production wells. Consequently the avoidance of the formation of the heavier layer is advantageous. Several methods for the introduction of heat have been proposed. The methods include surface heating by indirect heat exchange between the solvent and a hotter fluid, and downhole heating by electrical means e.g. resistance heating, and electromagnetic heating such as radio frequency (RF) and inductive heating (IH). The methods are energy-intensive, expensive, and tend to create significant quantities of greenhouse gases. In-situ combustion by burning a portion of the native heavy oil production or a portion of the injected solvent has also been proposed, but it suffers from safety issues and operational challenges.
There is a need for an effective way of providing heat to solvent-dominated recovery processes.